The element V is added to materials in order to improve strength (yield strength and fatigue strength) of forged products or to omit heat treatments. It has, however, problems in the price fluctuation by resource depletion of V and its addition cost.
Therefore, a good strength is assured by conducting a forging process against a portion required to have a high-level strength, in a temperature range of from not higher than Ar1 transformation point to 200° C. during the cooling process after a hot forging (for example, see Japanese Patent Application Publication 2003-055714).
In case that the forging process is conducted in the blue shortness region, however, it is necessary to have a heat treatment for the embrittlement recovery. Therefore, it is difficult to reduce the production cost. On the other hand, in the case of applying a temperature range of not lower than 600° C., it becomes a heating close to Ar1 transformation point. Therefore, the previously obtained ferrite and pearlite texture starts growing, and pearlite grains start enlarging. Furthermore, the ferrite precipitation occurs in the pearlite grains from the strain after the process as a point of the origin. Therefore, it is difficult to obtain the target strength.